Basic mechanisms controlling orbital order and orbital fluctuations in transition metal oxides are discussed. The lattice driven classical orbital pi

We study the exchange interactions and resulting magnetic phases in the honeycomb cobaltates. For a broad range of trigonal crystal fields acting on ${\mathrm{Co}}^{2+}$ ions, the low-energy pseudospin-$1/2$ Hamiltonian is dominated by bond-dependent Ising couplings that constitute the Kitaev model. The non-Kitaev terms nearly vanish at small values of trigonal field $\mathrm{\Delta }$, resulting in spin liquid ground state. Considering ${\mathrm{Na}}_3{\mathrm{Co}}_2{\mathrm{SbO}}_6$ as an example, we find that this compound is proximate to a Kitaev spin liquid phase, and can be driven into it by slightly reducing $\mathrm{\Delta }$ by $\sim 20\mathrm{meV}$, e.g., via strain or pressure control. We argue that, due to the more localized nature of the magnetic electrons in $3d$ compounds, cobaltates offer the most promising search area for Kitaev model physics.

Complex oxides with 4d and 5d transition-metal ions recently emerged as a new paradigm in correlated electron physics, due to the interplay between spin–orbit coupling and electron interactions. Fo...

The discovery of high-temperature superconductivity in La 3 Ni 2 O 7 at pressures above 14 GPa has spurred extensive research efforts. Yet, fundamental aspects of the superconducting phase, including the possibility of a filamentary character, are currently subjects of controversial debates. Conversely, a crystal structure with NiO 6 octahedral bilayers stacked along the c -axis direction was consistently posited in initial studies on La 3 Ni 2 O 7 . Here, we reassess this structure in optical floating zone-grown La 3 Ni 2 O 7 single crystals that show signs of filamentary superconductivity. Employing scanning transmission electron microscopy and single-crystal x-ray diffraction under high pressures, we observe multiple crystallographic phases in these crystals, with the majority phase exhibiting alternating monolayers and trilayers of NiO 6 octahedra, signifying a profound deviation from the previously suggested bilayer structure. Using density functional theory, we disentangle the individual contributions of the monolayer and trilayer structural units to the electronic band structure of La 3 Ni 2 O 7 , providing a firm basis for advanced theoretical modeling and future evaluations of the potential of the monolayer-trilayer structure for hosting superconductivity. Published by the American Physical Society 2024

α-RuCl₃ is a major candidate for the realization of the Kitaev quantum spin liquid, but its zigzag antiferromagnetic order at low temperatures indicates deviations from the Kitaev model. We have quantified the spin Hamiltonian of α-RuCl₃ by a resonant inelastic x-ray scattering study at the Ru L₃ absorption edge. In the paramagnetic state, the quasi-elastic intensity of magnetic excitations has a broad maximum around the zone center without any local maxima at the zigzag magnetic Bragg wavevectors. This finding implies that the zigzag order is fragile and readily destabilized by competing ferromagnetic correlations. The classical ground state of the experimentally determined Hamiltonian is actually ferromagnetic. The zigzag state is stabilized by quantum fluctuations, leaving ferromagnetism – along with the Kitaev spin liquid – as energetically proximate metastable states. The three closely competing states and their collective excitations hold the key to the theoretical understanding of the unusual properties of α-RuCl₃ in magnetic fields.

We have investigated the excitation spectra of ${j}_{\mathrm{eff}}=\frac{1}{2}$ Mott insulator ${\mathrm{Na}}_{2}$${\mathrm{IrO}}_{3}$. Taking into account a relativistic multiplet structure of Ir ions, we have calculated the optical conductivity $$\sigma${}($\omega${})$ and resonant inelastic x-ray scattering (RIXS) spectra, which manifest different features from those of a canonical ${j}_{\mathrm{eff}}=\frac{1}{2}$ system ${\mathrm{Sr}}_{2}$${\mathrm{IrO}}_{4}$. Distinctly from the two-peak structure in ${\mathrm{Sr}}_{2}$${\mathrm{IrO}}_{4}$, $$\sigma${}($\omega${})$ in ${\mathrm{Na}}_{2}$${\mathrm{IrO}}_{3}$ has a broad single peak dominated by interband transitions from ${j}_{\mathrm{eff}}=\frac{3}{2}$ to $\frac{1}{2}$. RIXS spectra exhibit the spin-orbit (SO) exciton that has a two-peak structure arising from the crystal-field effect, and the magnon peak at energies much lower than in ${\mathrm{Sr}}_{2}$${\mathrm{IrO}}_{4}$. In addition, a small peak near the optical-absorption edge is found in RIXS spectra, originating from the coupling between the electron-hole ($e$-$h$) excitation and the SO exciton. Our findings corroborate the validity of the relativistic electronic structure and importance of both itinerant and local features in ${\mathrm{Na}}_{2}$${\mathrm{IrO}}_{3}$.